1. Field of the Invention
The present invention relates generally to television signal processing and more particularly to a method and apparatus for estimating a level within the blanking interval of a television signal.
2. Background Information
Cable television terminals, commonly referred to as settops, require that analog channel video in the form of a composite video signal, such as the video signal conforming to the standard promulgated by the National Television Systems Committee (NTSC), be digitized and decoded to component form, in order to enable features such as high quality graphics overlay and video scaling in the settop device.
Accurate rendition of the video image requires that the active video level be correctly determined. A preferred way to extract the correct active video level data from the composite video signal is to first determine the video blanking level and to thereafter, reference the active video level to the video blanking level. By first determining the video blanking level, the requirements imposed on the video clamp speed and on direct current level accuracy preceding the video decoder are reduced.
In realistic cable television installations, there can be substantial noise on the video signal caused by noisy channels, or low radio frequency (RF) input level to the settop. Noise occurring during the blanking interval can cause the recovered video blanking level to be inaccurate for a particular line and also, to change from line to line, resulting in inaccurate luminance values or substantial visual artifacts, such as horizontal striping.
A high quality video decoder also incorporates automatic gain control (AGC) to compensate for variations of the amplitude of the composite video signal. When AGC is performed by the video decoder, the decoder measures the difference between the amplitude of a sync tip and the blanking level of the composite video signal and adjusts the amplitude of the composite video signal to maintain a relatively constant value.
Conventionally, the blanking level is determined by averaging the level of the back porch portion of the blanking signal portion of the composite video signal subsequent to removing the color burst signal from the blanking signal by a band reject frequency filter. Alternatively, as is done by some digital techniques, a band reject filter is not used, and the blanking level is determined by sampling the level of the breezeway and/or color back porch portions of the back porch of the blanking interval. The former technique requires a costly filter band reject filter, while the latter technique is sensitive to noise.